The invention relates to an acoustic surface wave filter comprising at least one member which can be stimulated to surface oscillations and is made at least partly of piezoelectric material, the surface on at least one side of the member being provided with disturbance locations (possibly impurity locations) for surface waves, the filter also comprising means for converting electrical into mechanical energy and vice versa, at least some of the disturbance locations being combined into a resonator in the form of a ruled or line grating and the average distance between the disturbance locations in the resonator being equal to half the wave-length of the surface waves or an integral multiple thereof.
The transmission behaviour of acoustic surface wave filters is such that, without weighting the dimensions of the transducer fingers, the attenuation frequency varies approximately in accordance with (sin x/x ).sup.2, x being a linear function of the frequency. This transmission behaviour is similar, with regard to flank steepness, to that of a three-circuit band-pass filter. There are known methods of influencing this behaviour, either by weighting the geometry of the transducer fingers or by producing coupled resonance structures in the path travelled by the surface wave, so that the filter slopes are made steeper, like those of a multi-circuit band-pass filter. The disadvantage of this method is that accurate photo-etching is required to prevent diffraction effects.
Coupled resonance structures, which can likewise be used to improve the flank steepness, are described in German Offenlegungsschrift No. 2,133,634 published July 6, 1971. These resonance structures comprise disturbance locations disposed perpendicular to the propagation direction of the surface waves, the distance between adjacent spots being chosen so as to produce a resonator in conjunction with the intermediate portions of surface. As before, however, these structures have to be manufactured very accurately, since the accuracy with which the resonance frequency of a resonator can be tuned depends on the accuracy with which two adjacent disturbance locations satisfy the condition .lambda./2.
German Offenlegungsschrift No. 2,363,701 published June 26, 1975 describes an acoustic surface wave filter characterised by good flank steepness and adapted for making wide variations in the intensity of the proportion of oscillations transmitted by the filter. To this end, according to the last-mentioned specification, at least some of the disturbance locations are combined into a resonator in the form of a ruled grating and the average distance between disturbance locations is made equal to half the wave-length of the surface waves or to an integral multiple thereof.
As a result of the combined effect of many weak reflections on the grating lines, a standing wave distribution builds up for frequencies at which the distance between adjacent lines of the grating is .lambda./2 or an integral multiple thereof. In one form of grating of this kind, it is stimulated at its input, for example, at a short interdigital line, with a constant a.c. voltage, and at its output, for example, an interdigital line, it delivers a voltage U which varies in accordance with a resonance curve which exhibits a relatively narrow peak.
A complete ruled or line grating, therefore, has the properties of an individual resonator.
To obtain a filter having a wider pass-band, it is usual to couple a number of resonators together. Some advantageous methods of acoustically coupling resonators are described in detail in German Offenlegungsschrift No. 2,363,701.